Saskatoon berry / Juniper Rust

Causal Organism: Gymnosporangium nelsonii

Crops Affected: Saskatoon berry, juniper species (alternate host); also causes rust on pears and some other Rosaceous species
Disease Cycle:

• 2 hosts are required to complete the entire sexual cycle
  • The pathogen cycles between species
  • It may also increase as specific spore types on each host
• Galls on junipers produce jelly-like orange-brown “horns” in wet springs, releasing spores that infect the alternate species
  • These are typically produced in May and June
  • Spores can travel several kilometres to infect Saskatoon berry plants
• Leaves and fruit of Saskatoon berries may be infected, causing the characteristic swellings and growths
• Infection and spread is favoured by temperatures between 10-24°C, accompanied by wet plant surfaces
• Moist or rainy conditions can increase spore production and the spread of infection
• Spores produced on Saskatoon berries will infect junipers, completing the life cycle

Symptoms:

Junipers

• Globular woody galls which produce jelly-like spore bodies (horns) after rain

Saskatoon berries

• Early symptoms include yellowish spots and swellings on leaves and fruit
• Swellings grow to become firm spiky outgrowths from leaves and fruit
• Twigs and branches may swell and be distorted
• Orangey rusty powder will be evident on and around outgrowths

Management:

• Avoid planting near native stands of Juniper
• Remove junipers or prune out galls from infected junipers within approximately 1-2 km of orchards

• • This is fairly impractical in areas where rust is prevalent
• Apply registered protective controls during late May – mid June
  • Adhere to prescribed rates and pre-harvest intervals

Rust infected spur and fruit on Saskatoon berry

Photo by Tricia Simon